Technical Field
The present disclosure relates to an integrated microfluidic circuit with electrowetting-based operation and a corresponding microfluidic system.
Description of the Related Art
Known is the increasing use of integrated microfluidic circuits, for example manufactured with semiconductor materials and using MEMS (MicroElectroMechanical Systems) techniques, to obtain so-called “labs-on-chip”.
Lab-on-chip microfluidic analysis devices are used for carrying out chemical reactions on extremely small amounts of substances, for example, but not only, in the medical field, and in the field of diagnostics and follow-up.
Some advantages of the above devices consist in the considerable reduction of size and costs as compared to traditional solutions for laboratory analysis, in the speed of response, and in the reduction of the amount of specimens that are to be analyzed.
Integrated microfluidic circuits used in lab-on-chip devices include, for example, transport circuits, controlled-flow circuits, confinement circuits, circuits for mixing fluids, or, in general, circuits for actuation of one or more fluids.
It is also known that one of the major difficulties in manufacturing integrated microfluidic circuits lies in the integration of suitable actuators for movement of the fluids, such as valves, pumps, or actuation devices in general.
For the above purpose, some known solutions envisage the use of micromechanical actuators, i.e., ones based on mechanical deformations and movements on a micrometric or submicrometric scale. These solutions are not, however, altogether satisfactory, in particular as regards the complexity of implementation and the difficulties in the reduction of costs and size.
In order to overcome the above problems, other solutions that have been proposed envisage exploiting the so-called “electrowetting” phenomenon, i.e., the property of an electric field to modify the surface tension of a liquid, by introducing a term of electrostatic energy in the energy balance of the system constituted by the liquid and the surface of solid material with which the liquid is in contact.
In greater detail, and with reference to FIG. 1a, in the absence of an electric field, the contact surface between a supporting material, designated by 2, and a given amount of liquid 3, is hydrophobic. In other words, the contact angle θ between the liquid and the surface is greater than 90°.
Upon application of an electric field E (FIG. 1b), achieving a better condition in terms of energy balance entails a reduction of the value of the contact angle θ. In the case where the angle of contact θ is less than 90°, the contact surface 2 becomes hydrophilic, bringing about a “flattening” of the amount of liquid 3 on the contact surface 2, which is all the more marked the lower the value of contact angle θ.
Integrated microfluidic circuits based on the phenomenon described above in general envisage the use of a set of adjacent electrodes, which can be controlled individually from the electrical standpoint.
The electrodes define, on the surface of a supporting substrate to which they are coupled, fluidic paths, along which drops (or “packets”) of fluid are conveyed by selective turning-on or turning-off of adjacent electrodes. In fact, the local generation of electric fields by turning on the electrodes enables activation or deactivation of the electrowetting characteristics of the portions of substrate located at the same electrodes, which are each time rendered hydrophobic or hydrophilic, thus attracting or repelling the drops or packets of fluid.
Solutions for fluidic transport of the above sort are frequently defined as solutions of “digital microfluidics”, given the on/off characteristic of the achieved transport mode.
A detailed description of these solutions may be found, for example, at: http://microfluidics.ee.duke.edu, by Duke University, Durham, N.C.
Also these solutions are not, however, free from defects and problems.
In particular, the resolution (in terms of the minimum amount of fluid that can be moved) of a microfluidic circuit obtained in this way is dictated by the minimum size of the electrodes, which represent in general the “pixel” or elementary unit of movement of the fluid, thus determining the amount of the fluid and the spatial resolution of the circuit.
Moreover, each electrode is connected to an electrical supply source by means of an appropriate electrical connection element. This entails a considerable complexity of the resulting electrical connections, an increase in the occupation of area, the presence of geometrical constraints, and possibly an additional metal layer.
Consequently, it is difficult to provide complex designs for the microfluidic circuits (for example, for the preparation of specimens for being analyzed on chips having small dimensions), and in any case these circuits have a high occupation of area.
In addition, if a mixer for fluids is made with the solution described, the composition of the mixed fluid is a function of the ratio of a certain integer number m of drops (or packets) of a first fluid, and of a respective integer number n of drops (or packets) of a second fluid.
In particular, the minimum amount of fluid that can be mixed with a mixing ratio m:n is (m+n)·d, where d is the volume of an individual drop (or packet) of fluid, which is a function, as previously highlighted, of the characteristics of the electrodes.
Consequently, mixing of the fluids can be carried out only in a discrete way, which is not finely tunable, as a function of the volume d of the individual packet of fluid that can be moved.
Recent studies seem also to indicate that the same volume of fluid that can be transported by each pixel or elementary electrode unit is not always repeatable, precise, and constant over time, with consequent possible imprecision in the movement and management of the fluids.